Can I reach the World's horizon_color in bge?
I'm not sure if this is what you want:

import bge
scene = bge.logic.getCurrentScene()
# background
saveit = scene.world.backgroundColor
scene.world.backgroundColor = [r,g,b]
# ambient light
scene.world.ambientColor = [r,g,b] 


With r,g,b float values from 0 to 1.

If you have mist enabled, you should also do:

# mist color
scene.world.mistColor = [r,g,b]